1. Field of the Invention
The invention relates to a process for the regeneration of a catalyst, whereby said process operates in a degraded mode, (partial-regeneration) in the case where the combustion capacity of the regenerator is not sufficient to eliminate all of the coke that is deposited on the catalyst that is to be regenerated.
More particularly, this invention relates to the degraded-mode regeneration, or the partial-regeneration mode, of a catalyst that is used in moving-bed processes for the production of aromatic hydrocarbons, in particular the reforming or the dehydrogenation of paraffins. The catalyst that is used for the production of aromatic hydrocarbons, and in particular reforming, generally comprises a substrate (formed by, for example, at least one refractory oxide; the substrate can also include one or more zeolites), at least one noble metal of group VIII of the periodic table (Handbook of Chemistry and Physics, 76th Edition, 1995-1996), whereby this noble metal is preferably platinum and preferably at least a promoter metal (for example tin or rhenium), at least one halogen and optionally one or more additional elements (such as alkalines, alkaline earths, lanthanides, silicon, elements of group IV B of the periodic table, non-noble metals, elements of group III A, etc.). The catalysts of this type contain, for example, platinum and at least one other metal which are deposited on an alumina substrate; this alumina also undergoes chlorination before, after, or during the deposition of different elements. In general, these catalysts are used for the conversion of naphthenic or paraffinic hydrocarbons that are able to be transformed by dehydrocyclization and/or dehydrogenation, in reforming, or for the production of aromatic hydrocarbons (for example, production of benzene, toluene, ortho-, meta- or paraxylenes). These hydrocarbons come from fractionation of crude oils by distillation or other transformation processes. These catalysts are extensively described in the literature.
The catalysts that are used for the dehydrogenation of paraffins generally comprise a substrate that contains, for example, at least one refractory oxide such as alumina and at least one noble metal of group VIII of the periodic table; this metal is preferably platinum or at least an oxide of at least one element that is selected from the group that is formed by groups V and VI of the periodic table; these oxides are preferably chromium oxide, molybdenum oxide, or vanadium oxide. These catalysts also often contain one or more additional metals, such as potassium and/or lithium.
One of the ways of increasing the yields of these processes for reforming or aromatic compound production or else dehydrogenation of paraffins is to reduce the operating pressures at which the different advantageous reactions are carried out. For example, 30 years ago reforming reactions were carried out at 40 bar; 20 years ago it was 15 bar. Today, it is common to see reforming reactors that operate at internal pressures of 10 bar, in particular between 3 and 8 bar.
The enhancement of beneficial reactions caused by pressure reduction is accompanied by faster deactivation of the catalyst by coking. Coke, which is a compound of a high molecular weight that consists essentially of carbon and hydrogen, is deposited on the active sites of the catalyst. The H/C molar ratio of the coke that is formed varies from about 0.3 to 1.0. The carbon and hydrogen atoms form condensed polyaromatic structures whose percentage of crystalline organization varies depending on the nature of the catalyst and operating conditions of the reactors. Although the selectivity of the transformation of the hydrocarbons into coke is very low, the coke contents that accumulate on the catalyst can be significant. Typically, for fixed-bed units, these contents are often between 2.0 and 20.0 to 25.5% by weight. For circulating-bed units, these contents are generally less than 10.0% by weight.
The deposition of coke, which is faster at low pressure, requires a regeneration of the catalyst that is also faster. The current regeneration cycles can drop to 2-3 days.
2. Description of the Prior Art
The regeneration of these catalysts has already been described in the prior art; generally these regenerations are carried out according to three essential stages that are implemented in three corresponding zones. A combustion stage is thus carried out during which the coke is eliminated by burning with an oxygen-containing gas, a halogenation stage in which the catalyst is flushed with a halogen gas that contains water and that makes it possible to reintroduce halogen into the catalyst and to redisperse the metal phase, and a drying zone or calcination zone in which the water that is produced by the combustion of the coke as well as the water that is provided by the halogenation gas is eliminated. This 3-stage scheme is very often followed by a reduction stage during which the catalyst is reactivated. By regeneration, the user seeks to obtain a catalyst whose properties come as close as possible to those of the new catalyst; it is decoked, and the metal phase is redispersed. In prior art, the catalyst from combustion contains generally less than 0.1% coke.
Such processes are extensively described in the literature and, for example, in Patents U.S. Pat. No. 4,980,325, U.S. Pat. No. 5,053,371, EP-A-378482, and Patent Application FR 97/04660 from the applicant.
U.S. Pat. No. 4,980,325 describes a process for continuous or semi-continuous regeneration of a deactivated reforming catalyst. The process is carried out according to the three standard stages that are described above with a catalyst that comprises a metal of group VIII of the periodic table and a halogen that is deposited on a substrate with an alumina base. The combustion stage is carried out in the presence of a recycling gas whose molar oxygen concentration is about 0.5 to 1.5%, and the vent gas from this stage is partly evacuated and partly recycled to this combustion zone after being enriched with a so-called "oxygenpoor" gas.
This so-called "oxygen-poor" gas comes from a stream of air that has been divided into, on the one hand, an oxygen-poor flow--i.e., its oxygen concentration is less than 12% by volume--and, on the other hand, an oxygen-rich flow--i.e., its oxygen concentration is greater than that of air.
After the combustion stage, the catalyst undergoes a halogenation stage and then a calcination stage, in which said catalyst is brought into contact with a mixture that consists of a portion of the oxygen-rich flow, combined with air.
U.S. Pat. No. 5,053,371 describes a process for continuous regeneration of a reforming catalyst that is carried out according to the three standard stages that are described above. This process comprises a combustion stage in which the catalyst is brought into contact with an oxygen-containing recycling gas. This recycling gas is evacuated from the combustion zone, a fraction of this gas is evacuated from the device, and the other fraction is sent into a combustion recycling loop there is added. In the recycling loop, a mass of oxygen-containing gas that is equal to the mass of the fraction of this gas that is evacuated from the device.
The catalyst then undergoes a halogenation stage and then a calcination stage with an oxygen-containing gas. In the halogenation stage, a portion of the halogenation gas is recycled, and a halogen is added to this halogenation loop to maintain the halogen concentration in the halogenation zone. The various gases that circulate in the zones of the regeneration device can circulate in part and move from the calcination zone to the halogenation zone and then from the halogenation zone to the combustion zone. An enhancement of this invention consists in measuring the concentration in the recycling gas to determine the amount of oxygen-containing gas that is required for regeneration, and then in adding the required amount of oxygen-containing gas to the halogenation loop.
Patent EP-A-378,482 from the applicant discloses a process for continuous regeneration of a reforming catalyst or a catalyst for aromatic compound production that makes it possible to overcome the drawbacks that are inherent in increasingly short regeneration cycles. One of the stages of regeneration is the oxychlorination of the catalyst. According to Patent EP-A-378,482, the used catalyst gradually advances from top to bottom in a regeneration chamber where it successively encounters a first zone with a radial moving and combustion bed, a second zone with a radial moving and combustion bed, a zone with an axial oxychlorination moving bed, and a zone with an axial calcination moving bed, wherein
(a) in the first combustion zone, the catalyst is treated by a combustion gas with an inert gas base that circulates in co-current with the catalyst and that contains 0.01 to 1% by volume of oxygen, whereby this combustion gas is obtained from a zone for scrubbing the gases that are obtained from combustion, oxychlorination, and calcination. PA1 (b) in the second combustion zone, the catalyst is treated in the presence of the gases that come from the first combustion zone and in the presence of an inert make-up gas to which is added up to 20% oxygen by volume so that the catalyst is in contact with a gas that contains 0.01 to 1% oxygen by volume; PA1 (c) the waste gases are evacuated from the second combustion zone and are sent to a scrubbing loop after having been previously mixed with gases that are drawn off from the oxychlorination zone and the calcination zone. PA1 each combustion zone is separated from the adjacent combustion zones in order to be able to allow the catalyst to pass and to prevent the passage of the gases, PA1 at least one oxygen-containing gas is introduced into each zone of the combustion stage, and the gases that are produced are extracted from each zone; the harshness of the operating conditions in each zone of the combustion stage increases with the direction of the flow of the catalyst.
Patent Application FR 97/04660 describes a process for the regeneration of a reforming catalytic moving bed or for aromatic hydrocarbon production, or else for dehydrogenation of paraffins that contain a substrate, at least one noble metal, and at least one halogen; this process comprises a combustion stage with treatment of the catalyst in a moving bed in at least two successive combustion zones, an oxychlorination stage and a calcination stage, in which